(1) Field of the Invention
The invention relates to a method for controlling a pneumatic brake system and for operating a pneumatic brake system, and such a pneumatic brake system for a vehicle.
The method is provided, in particular, for carrying out an automatic control process or an automatically initiated control process.
(2) Description of Related Art
Electropneumatic brake systems generally have an electronic control device (ECU), pneumatic wheel brakes, compressed air accumulators for a plurality of brake circuits and suitable electropneumatic and pneumatic valves.
In what are referred to as EBS systems, the brake pressure which is output to the pneumatic wheel brakes is modulated electrically by means of the electronic control device; if appropriate it is possible to provide a pneumatic fallback level in case the electrical actuation fails. However, such EBS systems are generally relatively complex.
Electropneumatic brake systems without such completely electronic actuation are also referred to as ABS systems and generally have, immediately upstream of the wheel brakes, check valves, also referred to as ABS check valves, and also a pilot-control valve device for actuating the ABS check valves. The ABS check valves are actuated by the pilot-control valve device and can then assume their various operating phases; these are                increasing the pressure, i.e. an open position,        maintaining the pressure, i.e. a closed position, and        reducing the pressure, which can take place in a bleeding position by bleeding the brake lines to the wheel brakes, or else in an open position by reducing the pilot-control pressure via the pilot-control valve device, in order to set the brake pressure at the wheel brakes. In this context, generally direct actuation of the ABS check valves is additionally possible via a brake valve which is connected to the brake pedal.        
DE 10 2011 115 128 B4 presents such a brake system and a method for controlling or operating such a brake system. In this context, a pressure is input into wheel brakes which are switched on and off alternately via electrical actuation of the control device, in order to set a desired braking behavior, in particular for ACC (automatic cruise control) as an automatic control method.
DE 102 32 792 A1 describes a braking method for a vehicle which is equipped with an anti-lock brake system (ABS) and a traction control system (TCS) as well as also automatic braking means such as an ACC system and a rollover prevention system (RSC). In this context, a brake pressure distribution occurs between driven and non-driven axles. The brake pedal passes on pressure to ABS check valves of the wheel brakes via a brake valve.
DE 10 2009 053 815 B4 describes a service brake device of a vehicle, in which service brake device a test run for the valves is provided. The brake system has a pressure-inputting 3/2 way valve, brake cylinders and ABS pressure control valves, each with an inlet valve and an outlet valve.
EP 844 155 B1 describes a further pneumatic EBS brake system.
Furthermore, basically methods for actuating valves by means of pulse width modulation (PWM) are known. DE 196 106 65 C2, EP 134 7908 B1 and U.S. Pat. No. 5,815,362 A1 describe specific PWM signals.
PWM actuation permits energization by means of periodic switching on and off of the current. In this context, a pulse frequency or period is predefined in which the pulse width of the switch-on pulse and correspondingly the subsequent pulse width of the switch-off pulse can be varied, in order to be able to set currents or power levels between a minimum value and a maximum value. When electromagnetic valves or solenoid valves are actuated, an armature, which is to be adjusted, of the valve can therefore be held in a floating state, in order to continuously hold this central position between a minimum value and a maximum value. The PWM actuation therefore serves to replace a median voltage value, since median voltage values which are to be set in an analogous fashion can generally be produced only by means of, for example, potentiometer circuits or other circuits which are subject to loss.